[Keeping House] Split JSON tests into multiple files

[hudson-ai]

@riedgar-ms as per your suggestion, I have opened a separate PR for splitting the JSON tests into multiple files (and running black). I think we should probably go a step further, as I only moved the reference tests from the main test_json.py to test_refs.py so far.

Let's do that later so we can keep this minimal :)

P.S. I've already merged this locally with my other PR branch, so there will be no surprises or difficulties there.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.00%. Comparing base (b4b0047) to head (ed0a753).
Report is 1 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1072      +/-   ##
==========================================
- Coverage   65.35%   64.00%   -1.35%     
==========================================
  Files          65       65              
  Lines        4892     4892              
==========================================
- Hits         3197     3131      -66     
- Misses       1695     1761      +66     

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